1. Field of the Invention
This invention pertains to integrated optical circuits with optical waveguides having high index contrast and low optical loss for passive optical devices, thermooptic devices, and integrated optical circuits. More particularly, the invention pertains to the chemical vapor deposition of silicon-oxycarbide optical films and the formation of optical core elements and cladding layers to form optical waveguides and integrated thermooptic devices.
2. Description of the Related Art
Long haul, metropolitan, and local networks predominantly rely on optical communications for the transmission of data between nodes. More recently, optical data transmission has been used for computing and data processing systems to enable high bandwidth communications between processors, memory, and I/O. Many of the functions required by optical communications networks are currently handled by collections of discrete devices. Significant cost, size, and performance improvements may be obtained by shrinking optical waveguide circuitry and integrating multiple devices and optical functions on a single chip to form integrated optical circuits.
Optical waveguides are the key building block of integrated optical circuits. FIG. 1 depicts one typical planar optical waveguide 10 formed on a substrate or wafer 20. The lower optical cladding 30 is formed first. Next, optical core 40 is formed by chemical vapor deposition, lithographic patterning, and etching. Finally, the core 40 is surrounded with an upper optical cladding layer 50. An optical waveguide or combination of optical waveguides can be assembled to form devices such as ring resonators, arrayed waveguide gratings, couplers, splitters, polarization splitters/combiners, multimode interference (MMI) couplers, Mach-Zehnder interferometers, mode transformers, optical vias, and polarization rotators. These devices may then be combined on an optical chip to create an integrated optical device or circuit that performs one or more optical functions such as multiplexing/demultiplexing, optical add/drop, variable attenuation, switching, splitting/combining, tunable filtering, variable optical delay, clock distribution, and dispersion compensation.
The material system most commonly used for planar optical waveguide devices is germanium doped silicon oxide SiO2:Ge. The waveguide consists of a SiO2:Ge optical core element, having a first refractive index, that is surrounded by lower and upper cladding layers each, having smaller refractive indices. Typical cladding layer materials include silica (SiO2) and doped silicas such as phosphosilicate glass (PSG), borosilicate glass (BSG), borophosphosilicate glass (BPSG), and flourine doped silica (SiOF). The ability to shrink the dimensions of an optical circuit is generally limited by the refractive index contrast of the optical waveguides from which it is formed, where the refractive index contrast is defined as the ratio (ncore−nclad)/nclad where ncore is the refractive index of the core and nclad is the refractive index of the cladding. The smallest possible size of an optical device is constrained by the minimum allowable radius of curvature of its optical waveguides before incurring significant optical propagation loss of 0.5 dB/cm or more. The minimum allowable radius of curvature is inversely proportional to the refractive index contrast. Thus, the higher the index contrast, the smaller the curvature of the waveguide, and hence the smaller the device, can be. For SiO2:Ge based optical waveguides, the maximum index contrast is limited to about 0.03 (3%), resulting in a minimum radius of curvature of at least 500 μm. As an example, a single ring resonator waveguide structure formed in this material system would at best consume approximately 1 mm2 area.
Silicon-oxynitride (SiON) is another doped silica that has been used for the fabrication of planar lightwave circuits (see, e.g., G. L. Bona, R. Germann, and B. J. Offrein, “SiON high refractive index waveguide and planar lightwave circuits,” IBM J. Res. & Dev. 47 (2/3), 239 (2003) incorporated herein by reference). Low loss optical waveguides have been demonstrated having SiON core elements and SiO2 cladding layers with significantly higher refractive index contrasts than is possible with SiO2:Ge (see, e.g., U.S. Pat. No. 6,614,977 and International Publication No. WO0164594, both incorporated herein by reference). Optical waveguides with refractive index contrasts of about 0.17 (17%) can be fabricated from SiON core elements, having a refractive index of 1.7, and SiO2 cladding layers, having a refractive index of 1.45 measured at a wavelength of 1550 nm. At this high index contrast, waveguides can be designed with radii of curvature as small as about 40 μm. For the SiON core elements, the highest potential refractive index value is 2, corresponding to silicon-nitride (Si3N4).
A common optical element in most optical integrated circuits is a thermooptic device, wherein a portion of the optical core waveguiding elements within the device is locally heated with respect to the rest of the device by use of a local resistive heating element. Local heating of a waveguide shifts the phase of an optical signal within the waveguide by way of the thermooptic effect on refractive index (dn/dT). In addition, there can be a secondary contribution to the thermally induced phase shift resulting from thermal expansion of waveguide dimensions.
A primary factor contributing to the overall performance of an integrated optical circuit containing thermooptic devices is power consumption. Power consumption affects the cost of optical systems through footprint, thermal dissipation, airflow, peltier cooler, and power supply requirements. The power consumption of a thermooptic device depends on several factors including device size as well as dn/dT. By increasing the refractive index contrast, the overall device size can be reduced to minimize the active area that must be heated. Even more significantly, increasing the material dn/dT lowers the temperature rise needed to change the refractive index by a given amount. For example, an optical device such as a ring resonator has a set of resonant wavelengths given approximately by:λi=2 πr n/iwhere r is the ring radius, n is the optical waveguide effective index, and i is an integer. The resonant wavelengths of the ring may be changed by locally raising or lowering the temperature of the ring waveguide, and the effective refractive index may be approximated byn=no+(dn/dT)ΔTwhere no is the effective index at temperature To, dn/dT is the rate of change of refractive index with temperature at To, and ΔT is the net change in temperature T−To. For optical devices formed from SiO2:Ge, the dn/dT is typically just under 1×10−5/° C., and for optical devices formed from chemical vapor deposited SiON-based optical waveguides with core element refractive indices of 1.7, the dn/dT is typically about 1.2×10−5/° C. By maximizing the dn/dT, a smaller temperature difference is needed to effect a specific change in refractive index. As a result, the optical power requirement of a device is minimized.
Silicon-oxycarbide (SiOC) is another doped silica that has recently been investigated extensively for electronic applications as an interlayer dielectric (ILD). SiOC films deposited using chemical vapor deposition (CVD) with an organosilicon precursor can be used to form a lower dielectric constant ILD than standard SiO2 (see, e.g., U.S. Pat. No. 6,627,532; U.S. Pat. No. 6,593,655; and H. J. Kim, Q. Shao, and Y. H. Kim, “Characterization of low-dielectric-constant SiOC thin films deposited by PECVD for interlayer dielectrics of multilevel interconnection,” Surface and Coatings Technology 171, 39 (2003) incorporated herein by reference). The mechanism by which the dielectric constant is reduced below that of SiO2 is through the replacement of oxygen atoms with terminating hydrocarbon groups from the organosilicon precursor and the subsequent decrease in film density. The CVD process and SiOC materials developed for electronic applications, however, do not pertain to high index contrast optical waveguides as the refractive index of these materials decreases along with the dielectric constant. The resulting SiOC films have a refractive index less than that of SiO2 and are not suitable for use as high refractive index optical core elements.